"The world of pure spirits"

“GOD’s universal providence works through secondary causes. . . . The world of pure spirits stretches between Divine Nature and the world of human beings; because Divine Wisdom has ordained that the higher should look after the lower, angels execute the divine plan for human salvation: they our guardians, who free us when hindered and help to bring us home.”

~St. Thomas Aquinas: Commentary on the Sentences, 2, 11, 1, 1.

Linaioli Tabernacle (detail), by Fra Angelico.
Tempera on wood, c. 1433;
Museo di San Marco, Florence.